I spent a couple of years growing up in Los Angeles, and I can still remember the feeling I'd get in my chest on particularly smoggy days in the summer. It was so bad some days that my brother and I would have to cut our Wiffle ball games short. If you live in a part of the country where the air quality isn't great, use iPhone app Smoggy (99 cents) to get a reading of the air quality before heading out to exercise or sending your kids out to play.
The app lets you use your current location or enter a zip code or city, and you can add multiple locations. For each location, it shows you the Air Quality Index, Particulate Matter, and Ozone. Tap on any of these three readings to get a description of what it is measuring. Smoggy can also be used as a basic weather app; below the three air quality readings it shows you the current temperature, chance for precipitation, and wind direction and speed. The app also gives you a basic air quality forecast -- good, moderate, unhealthy, and so on -- for the following day.
When viewing the current conditions of a city, you can swipe up to view the top 10 worst and best cities.
Smoggy gets its information from airnow.gov and more than 470 air quality stations around the U.S.